Conventionally, in order to achieve a high level of automated operation in a plant, a factory, or the like, a distributed control system (DCS) has been implemented in which on-site devices (measuring instruments and actuators) referred to as field devices are connected to a control apparatus that controls the devices by a communication means. Although such a distributed control system has almost always communicated by cable, recent years have seen the implementation of communication systems that communicate wirelessly, in conformance with an industrial wireless communication standard such as ISA100.11a or WirelessHART (registered trademark).
Because wireless communication has a higher probability of communication errors occurring than in cable communication, transmitted data might be lost. If data is lost, retransmission processing of the lost data is done, and, when retransmission processing is repeatedly done, a very long delay might occur from the start to the completion of data transmission. Furthermore, because the delay time caused by this retransmission is dependent upon the number of data retransmissions, the delay time is not fixed, but rather varies greatly. When such data loss and delay time variation occur, it can be imagined that problems might occur in process control.
PTL1 noted below discloses a network control system that implements stable plant control, taking into consideration of data loss (packet loss) occurring in wireless communication. Specifically, the system disclosed in PTL1 noted below implements stable plant control, without a great disturbance of the plant state, by correcting actuation amounts at the controller side, even if a situation occurs in which an actuator, which is a type of field device, cannot receive an actuation amount from a controller due to the packet loss.
In the above-described distributed control system, various state quantities (for example, pressure, temperature, and flow amount) in an industrial process are controlled by feedback control. Feedback control in a distributed control system is a control scheme in which a control apparatus acquires measurement signals (feedback signals) of state quantities measured by field devices (measuring instruments), and in which the control apparatus controls the above-noted various state quantities by actuating field devices (actuators) so that the acquired feedback signals coincide with target values.
In a distributed control system performing such feedback control, loss of data indicating measurement results of field devices (measuring instruments) or data indicating actuation amounts with respect to field devices (actuators), risks erroneous operation occurring in the feedback control. If variation occurs in the data delay time, there is a risk that wasted time in the feedback control becomes large, and erroneous operation occurs in the feedback control. In this case, the term wasted time refers to the time from the output from a control apparatus of an actuating signal to a field device (actuator) until the effect thereof appears in the feedback signal.
Using the art disclosed in the above-described PTL 1, because an actuation amount of a field device (actuator) is corrected even if a packet loss occurs, it is possible to implement stable process control. However, in the above-described PTL 1, it becomes necessary to perform a complex computation in order to correct the actuation amount of a field device (actuator), and there is a risk of the load on the control apparatus becoming large.